On the habits of polyethylene crystals

Abstract

This paper presents a full account of the crystal habits of chain-folded polyethylene crystals grown from dilute solutions, especially in xylene. All the growth procedures given are reproducible and have been used in the preparation of specific crystal habits discussed previously. An important innovation in technique is filtration at the crystallization temperature to avoid further crystallization occurring on cooling. In isothermal crystallization experiments this ensures the uniformity of fold length which has been confirmed to be a unique function of crystallization temperature, independent of concentration and molecular weight. A previous anomaly of double spacing has been removed, the equivalence of long spacing and layer thickness has been demonstrated and some more accurate figures for the latter have been obtained. Systematic variations of habit with temperature are recorded. At successively lower temperatures, truncated lozenges, true lozenges and dendrites form, dendrites being favoured by increasing molecular weight. Crystal growth experiments with changing temperature, including seeding experiments, have revealed that a change of crystallization temperature produces an apparently discontinuous change of layer thickness, a fact which is of basic significance for theoretical developments. Change in growth temperature also results in change in habit. As in polymers this means a change in structure, a new class of problem arises when different structures have to join within the same crystal boundary. By considering this problem, a number of growth features can be accounted for, e.g. the formation of thinner borders with particular habits, etc. The same considerations have been applied to the matching of microsectors in serrated and dendritic crystals, which is discussed, together with the possible growth mechanisms. Several unusual habits, e.g. special twinned, twisted and skeletal crystals, have been encountered during this work. The preparation of these has also been put on a systematic basis.